Magnetic recording hard disk drives with patterned magnetic recording media have been proposed to increase data density. In patterned media, the magnetic recording layer on the disk is patterned into small isolated data islands such that there is a single magnetic domain in each island or “bit”. The single magnetic domains can be a single grain or consist of a few strongly coupled grains that switch magnetic states in concert as a single magnetic volume. This is in contrast to conventional continuous media wherein a single “bit” may have multiple magnetic domains separated by domain walls. To produce the required magnetic isolation of the patterned islands, the magnetic moment of the spaces between the islands must be destroyed or substantially reduced so as to render these spaces essentially nonmagnetic. Alternatively, the patterned media may be fabricated so that that there is no magnetic material in the spaces between the islands.
Like conventional non-patterned or continuous-media disks, patterned-media disks also have non-data servo sectors that are used for read/write head positioning. The non-data servo sectors in patterned-media disks contain discrete islands separated by nonmagnetic spaces. The servo islands are patterned into a position error signal (PES) field that generates a servo readback signal that is demodulated into a PES for positioning the read/write head to the desired data track and maintaining it on track.
Several techniques have been proposed for fabricating patterned-media disks, including conventional lithography, direct-write electron-beam (e-beam) lithography, nanoimprinting, and guided self-assembly. These techniques are described in numerous references, including Terris et al., “TOPICAL REVIEW: Nanofabricated and self-assembled magnetic structures as data storage media”, J. Phys. D: Appl. Phys. 38 (2005) R199-R222. In nanoimprinting, a master template is fabricated, typically by direct e-beam writing, to have the desired pattern of discrete islands. The master template is pressed against a resist film on the disk substrate and subsequent etching steps of the substrate result in a patterned disk substrate onto which the magnetic layer is deposited. In guided self-assembly, a substrate is topographically patterned or the substrate surface is selectively chemically modified so that nanostructures can form in some areas and not others. Self-assembling block copolymers have been proposed for creating periodic nanometer-scale features that can be used to form the discrete islands. In guided self-assembly, the resulting discrete islands are typically formed as a hexagonal-close-packed (HCP) array. There are numerous references describing self-assembling block copolymers, including U.S. Pat. No 7,347,953 B2 and Kim et al., “Rapid Directed Self-Assembly of Lamellar Microdomains from a Block Copolymer Containing Hybrid”, Proc. of SPIE Vol. 6921, 692129, (2008).
Patterned-media disks, especially those with self-assembled HCP arrays of discrete islands, present a unique problem in servo-writing. Because the discrete islands are formed during a separate disk fabrication process, when the disks are mounted on the rotatable spindle of the servowriter (or the spindle of the disk drive if servowriting is done in the drive) the concentric data tracks can never be perfectly aligned with the center of rotation of the spindle. Also, the disk fabrication process may itself result in the data tracks not being perfectly concentric. Thus if the disk is rotated with the servowriter write head held at a fixed radial position from the center of rotation, the write head will typically traverse multiple tracks as the servo sectors pass the head during one disk rotation. This makes it impossible during the servowriting process for the head to magnetize the islands in the servo sectors according to the desired pattern.
What is needed is a method for servowriting patterned-media magnetic recording disks that have discrete magnetizable islands in the servo sectors that must be magnetized according to a desired pattern.